What are flexible printed circuits?

Flexible printed circuit boards (FPCs) are electronic circuit carriers based on a flexible, thin plastic material – usually polyimide, PEEK or polyester. Also known as flex PCBs or colloquially as flex boards, flexible printed circuit boards are designed to be permanently or temporarily bent, folded or integrated into curved geometries while retaining their electrical function. Their counterpart is rigid printed circuit boards.

The key feature of flexible printed circuit boards

As the name suggests, flexibility is the key feature of FPCs. Polyimide plastic forms the backbone of flexible printed circuits, offering a number of advantages over rigid printed circuits thanks to its heat resistance, flexibility and robustness. However, it is only with precisely guided conductor tracks that a reliable connection between all components can be achieved. It is important to arrange them in gentle curves rather than sharp bends. This is because tight angles create tension and thus potential weak points.

What are printed circuit boards used for?

Printed circuit boards serve to mechanically support electronic components and connect them electrically. They form the heart of almost all electronic devices – from smartphones and industrial equipment to medical technology and vehicle control systems. Depending on the application, printed circuit boards can be rigid, flexible or rigid-flexible.

Construction of flexible printed circuit boards

Flexible printed circuits consist of several functional layers that are carefully coordinated with each other. Each of these layers fulfils a specific task – be it mechanical support, electrical conductivity or protection against environmental influences. Based on the number of conductive copper circuit layers, FPCs can be divided into single, double or multi-layer types.

Single layer type

A single-layer flexible printed circuit board consists of a dielectric substrate, a single-sided copper layer and a protective layer on top. The conductor tracks are only on one side. This design is particularly thin, light and very flexible, which is well suited for simple, static applications with limited space.

Double-layer type

In a double-sided configuration, a copper layer is applied to both sides of the substrate. The two layers are connected to each other by vias, enabling more complex circuits to be created. However, flexibility is somewhat limited compared to the single-layer version.

Multi-layer type

Multilayer flexible printed circuit boards consist of several layers of copper and substrate material stacked alternately on top of each other. Vertical connections are made by internal or through vias. This construction allows for compact and powerful designs in a small space. However, the more layers are used, the stiffer the material becomes.

Special feature: Rigid-flexible printed circuit board

Rigid-flexible printed circuit boards consist of a combination of rigid and flexible areas. Flexible layers are embedded directly into rigid printed circuit boards. This design makes it possible to connect several rigid printed circuit boards to each other via flexible sections without additional wires or cable harnesses. This saves space, reduces plug connections and simplifies assembly.

What is the difference between rigid and flexible printed circuit boards?

Rigid printed circuit boards are made of solid, non-flexible materials such as fibre-reinforced epoxy resin (e.g. FR4). They retain their shape permanently and are used wherever mechanical flexibility is not required.

Flexible printed circuit boards, on the other hand, are made of thin, bendable plastic materials such as polyimide. They can be bent, folded or twisted – permanently or temporarily – without losing their function. This makes them ideal for tight installation spaces, mobile applications or devices with complex housing geometries. The main difference therefore lies in their mechanical flexibility and the resulting possible applications.

Can a printed circuit board be flexible?

Yes, that is possible. Flexible printed circuit boards are designed to adapt to a wide variety of geometries. Their thinness and mechanical flexibility make them easy to integrate into curved or compact device shapes. In rigid-flex circuits in particular, electronic components can be accommodated at different angles without losing the electrical connection. This results in space-saving solutions – without additional cables or connectors.

Risk of confusion: Semi-flexible printed circuit boards

Semi-flexible printed circuit boards should not be confused with ‘true’ flexible printed circuit boards. They are made of the same materials as rigid boards – usually fibre-reinforced plastic – and are made flexible by selectively thinning certain areas. However, the bending is only intended for assembly purposes. Semi-flexible printed circuit boards are less expensive and require larger bending radii.

Can printed circuit boards be lasered?

Yes, printed circuit boards can be processed very precisely with lasers. In modern electronics production, laser technology is used for cutting, drilling, structuring and removing layers, among other things. The laser offers decisive advantages, especially for flexible or rigid-flexible PCBs: It works without contact, burr-free and with high repeat accuracy – for example with machines from the DR20/30 series. Precise depaneling of flexible PCBs is another typical application for laser technology, especially in the final stage of series production.

Laser technology in FPC processing

As an experienced manufacturer of laser systems, Photonics Systems combines in-depth process knowledge with sophisticated technology. Our machine solutions, such as PCB routing, PCB drilling and PCB structuring, enable high-precision, reproducible results in the micro-machining of printed circuit boards – and are optimally tailored to the requirements of FPC manufacturing.

How are flexible printed circuit boards manufactured?

The manufacture of flexible printed circuit boards differs fundamentally from that of rigid circuit boards. Whereas parts of the metallic coating are removed from the fibre-reinforced plastic carrier layer in rigid circuit boards (e.g. by etching, milling or lasering), the copper conductor tracks in flexible circuit boards are first laminated onto a thin, flexible carrier film or applied directly.

Structuring and layer construction

In the next step, the copper is structured using photolithography to create conductor tracks, contact pads and signal connections. The protective layer (coverlay) above is opened at specific points to expose solder joints. A special adhesive material bonds all layers of material together through lamination, giving the structure stability and protecting it from moisture, dust and other environmental influences.

Corrosion protection and multi-layer structure

As with conventional circuit boards, the exposed copper on flexible circuit boards must be protected from corrosion by means of a suitable surface finish (with tin or gold). In more complex structures with multiple layers, additional through-hole plating (PTH) is used to ensure electrical connections between the copper layers.

Advantages and challenges of flexible printed circuit boards

Thanks to their flexible structure, compact design and high connection density, flexible printed circuit boards enable a significant reduction in space, weight and often also costs – especially when compared to equivalent rigid solutions.

Strengths:

• High flexibility for complex or three-dimensional designs
• Space and weight savings compared to rigid printed circuit boards
• Better vibration and temperature resistance
• Reduced installation effort due to elimination of cables and connectors
• Lower susceptibility to errors in dynamically stressed applications
• More reliable than cable harnesses, as FPCs can carry more current than wires

Weaknesses:

• Higher costs for development, prototyping and manufacturing
• Repairs and modifications are costly
• More sensitive to improper handling during assembly
• Not all manufacturers offer FPC production
• Only economically viable for medium to large quantities

Are circuit boards and printed circuit boards the same thing?

Yes, circuit board and printed circuit board are often used synonymously, but technically they do not mean exactly the same thing. A printed circuit board refers to the unassembled circuit carrier with conductor tracks – in other words, the bare carrier material. The term circuit board, on the other hand, is usually used when electronic components are already mounted on the printed circuit board. In everyday language, the differences are blurred, but technically the distinction makes sense.

Where are flexible printed circuits used?

Flexible printed circuits are used in many applications where rigid printed circuits reach their limits or need to be replaced. Thanks to their flexibility and thinness, they can be integrated into tight spaces and equipped with a wide variety of components. These properties make FPCs ideal for use in modern electronic devices.

They also replace conventional cables and connectors in dynamic applications, such as connecting a moving print head in an inkjet printer. In addition, they enable components to be arranged in a decentralised manner, for example by directly integrating additional sensors into the layout.

The future of flexible printed circuit boards

One particularly promising area of application is the automotive and aviation industries. Here, flexible printed circuit boards could replace the complex and heavy cable harnesses that often stretch for several kilometres today. For a long time, the limited production length (around 610 mm) was considered a technical obstacle – but modern manufacturing processes now enable significantly larger formats. At the same time, increasing competition is also driving down costs.

Conclusion:

Flexible printed circuit boards offer a technically attractive solution for applications with high requirements in terms of compactness, mobility and reliability. They enable space-saving designs, replace conventional cable connections and open up new design possibilities – especially in dynamic or complex systems. Although their manufacture is more complex and cost-intensive than that of rigid circuit boards, the benefits outweigh the disadvantages.